1. Field of the Invention
The invention relates to simple methods and devices to pulse ions into the flight tube of a time-of-flight mass spectrometer.
2. Description of the Related Art
Time-of-flight mass spectrometers with orthogonal injection of ions (abbreviated “OTOF”) usually are built with pushers which pulse a part of a fine beam of ions orthogonally to its original flight direction into the flight tube of the mass spectrometer. To generate the fine beam of ions, the ions usually are stored in a linear radio frequency (RF) ion trap, having their kinetic energy damped, and accelerated by a lens-type accelerator with a low voltage in the range of three to ten volts. This type of operation has a severe disadvantage: when the distance between accelerator and pusher has been crossed by heavy ions in the range of tens of kilodaltons, and the pusher has been filled with these heavy ions, light ions in the range of a few hundred Daltons have flown about ten times the distance, and their concentration within the pusher appears to be diluted by a factor of ten. This operation shows a strong mass discrimination.
To avoid mass discrimination, ions can be pushed out of a storage device directly into the flight tube of the mass spectrometer. It is known for about two decades that ions can be pushed out of a linear RF rod system in such a manner that the ions leave the rod system normal to the axis of the rods through one of the gaps between the rods into the flight tube of the mass spectrometer (see, e.g., U.S. Pat. No. 5,763,878, J. Franzen). This method did not become accepted in mass spectrometric practice, because the resulting dipolar ejection was not very exact, and resulted in low mass resolution.
Experience has shown that the ejection by the dipolar field is critical. When the dipolar ejection field is not a truly homogeneous field without any superposition of higher order fields, the mass resolution is degraded. In Patent Application Publication US 2013/0009051 A1 (M. A. Park) pushing devices for time-of-flight mass spectrometers are presented which allow for switching over between almost ideal quadrupole fields (for storing the ions) and almost ideal dipole fields (to push out the ions). This publication shall be incorporated herein by reference in its entirety.
The device of US 2013/0009051 A1, however, consists of a high number of electrodes around the storage volume, hard to build and hard to supply precisely with the high number of voltages required.